Magneto-resistance is the property of a material or a conductive structure to change the value of its electrical resistance in response to an applied external magnetic field. The effects involved include giant magneto-resistance (GMR), tunnel magneto-resistance (TMR) and anisotropic magneto-resistance (AMR). This property is typically used in spintronic devices, such as magnetic sensors and magnetic memory devices.
Early memory storage devices used read heads based on the AMR effect, in which the electrical resistance of a magnetic material depends on whether the magnetization is perpendicular or parallel to the electric current. Subsequent introduction of devices based on the GMR and TMR effects allowed a considerable down scaling of memory elements. The GMR based devices use a combination of at least two different ferromagnetic layers and a normal metal, while TMR based devices have an insulating layer between the ferromagnetic layers. In such structures, the relative parallel or anti-parallel alignment of the magnetization in the ferromagnetic layers leads to a difference in the electrical resistance of the structures.
In spintronic devices, the electric resistance is highly dependent on magnetic field or magnetization. In its basic form, the most common spintronic element is composed of two ferromagnetic electrodes, separated by a non-magnetic metal. When a certain voltage is applied across this structure, a parallel (anti-parallel) alignment of magnetization in the ferromagnetic electrodes results in a high (low) current through the structure due to the GMR effect, creating a spin-valve. Such a spin-valve is based on complicated fabrication techniques to allow a flipping of magnetization direction in one electrode while preserving the magnetization of the other electrode. For GMR based devices, the change in resistance with magnetic field ranges from 5-10% for simple structures to over 100% for complicated multi-layered structures. The sensitivity of GMR devices is orders of magnitude larger than in earlier AMR-based commercial devices, exhibiting magneto-resistive variations below 5%. AMR devices consist of a single ferromagnetic layer, displaying low (high) resistance when the magnetization of the layer is perpendicular (parallel) to the current across the layer. Despite the structural simplicity of AMR devices with respect to GMR devices, the relatively small change in resistance (1%-2% for simple ferromagnetic metals) places a severe limitation on miniaturization of AMR based devices to the characteristic micrometer size of current GMR devices. In such scales, the size of the AMR effect is comparable to the background noise and cannot be used for efficient data storage.
Spintronic devices are also used for high performance magnetic-random access memory (MRAM). For this purpose, the TMR effect is used in ferromagnet-insulator-ferromagnet layered structures, to achieve magneto-resistance of about 30% for tri-layered structures and up to 70% for more complex devices.
In order to further increase the sensitivity of GMR- and TMR-based memory elements, sophisticated production techniques, as well as expansive materials and elaborate multi-layered architectures, are required. In addition, TMR based elements require the use of complicated electronics to amplify the low signal. As for the MRAM elements based on the TMR effect, although making use of such tunnel junctions allows the design of a compact MRAM element, the high resistances of TMR devices result in very low electric signals that require further processing.
Thus, while impressive, the known GMR and TMR devices rely on unique properties of expensive metals and on the fabrication of complex multi-layered structures. The goal of the fabrication processes is two-fold: first, the GMR and TMR effects require some part of the ferromagnetic elements to be magnetically pinned. This allows one to switch from a parallel to anti-parallel magnetization configuration of the electrodes by application of a magnetic field, thereby affecting the scattering or tunneling properties of the charge carriers; second—an increase of the spin polarization of the current is needed, in order to minimize parasitic background current which is insensitive to magnetic manipulation. In addition, for TMR based devices the very small absolute signals require complicated electronic schemes for amplification and noise reduction.
There is a need in the art for a novel magneto-resistive structure, capable of amplifying magneto-resistive effects of electronic devices while retaining a large signal, without the need for rare metals or a complicated device structure.
A novel general approach is needed in order to enhance the sensitivity of spintronic elements, avoid complicated and costly fabrication processes, and maintain a high absolute signal.